This invention relates to the automatic control of distillation columns. More specifically, the invention relates to the control of such distillation columns having not only a feed input, overhead and bottom outlets, but also a sidedraw line. The invention relates both to the process and the apparatus aspects of this technology.
With computers and particularly microprocessors becoming more widely accessible and economically useful, a substantial development of automatic controls in the petrochemical industry has taken place. Especially mini-control systems for distillation columns have been developed. One such development is described in the U.S. Pat. No. 3,294,646. This patent describes the automatic optimization of bottoms and overhead flow of a distillation column by manipulating the heat input into the reboiler or the feed location (feed tray number) responsive to a control signal automatically computed from an analysis of the feed stream and other process variables. Whereas this patent is regarded to constitute a significant contribution to the art, it contains no solution of controlling a sidedraw stream from a distillation column so that the sidedraw is of approximately constant composition. This special problem arises in modern deisobutanizers, for instance.